Two-dimensional (2D) Nb-based oxynitrides are promising visible-light-responsive photocatalysts for the water splitting reaction, but their photocatalytic activity is degraded by the formation of reduced Nb5+ species and O2- vacancies. To understand the influence of nitridation on the formation of crystal defects, this study synthesized a series of Nb-based oxynitrides through the nitridation of LaKNaNb1-xTaxO5 (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0). During nitridation, K and Na species volatilized, which helped transform the exterior of LaKNaNb1-xTaxO5 into a lattice-matched oxynitride shell. Ta inhibited defect formation, yielding Nb-based oxynitrides with a tunable bandgap between 1.77 and 2.12 eV, straddling the H2 and O2 evolution potentials. After loading with Rh and CoOx cocatalysts, these oxynitrides exhibited good photocatalytic activity for H2 and O2 evolution in visible light (650-750 nm). The nitrided LaKNaTaO5 and LaKNaNb0.8Ta0.2O5 delivered the maximum H2 (19.37 μmol h-1) and O2 (22.81 μmol h-1) evolution rates, respectively. This work provides a strategy for preparing oxynitrides with low defect densities and demonstrates the promising performance of Nb-based oxynitrides for water splitting.
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